The present invention relates generally to radar-absorbing materials used for reducing detectable cross-section of an aircraft, such as a missile, and, more particularly, to a method for repairing such radar-absorbing materials.
Radar-absorbing materials are used to reduce detectability of aircraft, such as missiles. However, there are times where the radar-absorbing material is ablated or otherwise damaged, thereby rendering the aircraft more detectable by radar.
Currently, repairs to the radar-absorbing material are made with a two-part polyurethane (thermoset) to which a magnetic particle filler is added. The repair operator must (1) weigh out all three materials, which requires about 5 to 10 minutes, (2) mix half the filler into each part of the polyurethane, which requires an additional 10 to 15 minutes, (3) mix the two polyurethane parts together, which requires another 5 to 10 minutes, and (4) apply the material. The polyurethane must then cure for 5 hours before repair procedures can continue. After the polyurethane cures, the repair operator must sand the repair until smooth. This usually uncovers voids in the material that resulted from air entrapped during the mixing operation. The entrapped air is unable to rise to the surface before the material cures. If there are voids in the material after sanding, the repair operator must repeat the procedure to fill them. The repair time is 11-xc2xd hours for two applications.
One aerospace company demonstrated a hot melt RAM in 1995. The material remained very soft, and was not suitable for use. Further development was apparently discontinued.
Another aerospace company later came out with a hot-melt RAM as well, but did not demonstrate it. That material is no longer in use. It appears that there may have been adhesion problems.
Thus, a need remains for a rapid means of effecting repairs to radar-absorbing materials.
In accordance with the present invention, a glue-gun applied hot-melt radar-absorbing material and method of use thereof is provided which avoids most, if not all, of the prior art problems.
The hot-melt radar-absorbing material composition comprises:
(a) 70 to 85 wt % carbonyl iron powder;
(b) 2 to 10 wt % of a metal deactivator; and
(c) balance a thermoplastic polyurethane.
The method for repair of a body comprising a radar-absorbing material, comprises:
(a) formulating the hot-melt radar-absorbing material of the present invention;
(b) forming the hot-melt radar-absorbing material into a shape;
(c) applying the hot-melt radar-absorbing material in a molten state onto the body; and
(d) allowing the hot-melt radar-absorbing material to cool to room temperature.
In the present invention, the shape of the hot-melt RAM is advantageously a xe2x80x9cglue stickxe2x80x9d, which is configured to go into a glue gun. The repair operator loads the glue stick into the glue gun and pulls the trigger. The glue gun heats the glue stick, and the molten material is applied to the area to be repaired. The material hardens instantly on application. In contrast to the 11-xc2xd hours required by the prior art RAM, repair time is 22 minutes for two applications, employing the RAM of the present invention.
Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative embodiments are also briefly described as applicable.
In accordance with the invention, a hot melt RAM composition is provided. The hot melt composition comprises about 70 to 85 wt % of carbonyl iron powder, about 2 to 10 wt % of a metal deactivator, and the balance a thermoplastic polyurethane (one-component). The presence of the metal deactivator is required, due to the tendency of carbonyl iron to react with the polymer to break its backbone, thereby lowering the melt temperature to about 180xc2x0 F. (about 82xc2x0 C.). The metal deactivator prevents that reaction, and keeps the melt temperature of the polyurethane polymer in the range of about 350xc2x0 F. (about 177xc2x0 C.).
An example of a thermoplastic polyurethane is Elastolan C-95-A, available from BASF (Wyandotte, Mich.), which is a polyester-based polyurethane. The thermoplastic polyurethane, which is a one-component system, softens at about 350xc2x0 F. (about 177xc2x0 C.). Examples of other one-component, thermoplastic polyurethanes include the following from BASF (Wyandotte, Mich.): 1100 Series, 600 Series, C Series, and S Series; from Bayer (Pittsburgh, Pa.): Desmopan (polyester-based) series and Texin (polyester-based) series; from Dow (Midland, Mich.): Isoplas (polyether-based) series and Pellethane (polyester- and polyether-based) series; and from Stevens Urethane (Holyoke, Mass.): MP series.
Carbonyl iron powder is available from Byte Mark (Orlando, Fla.), Reade Advanced Materials (Riverside, R.I.), and Roschem Pacific Group (Australia).
An example of a metal deactivator is Irganox MD 1024, which is believed to be a hindered phenolic stabilizer, e.g., 1,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl) hydrazine, available from CibaGeigy (Tarrytown, N.Y.). Other examples of metal deactivators include Eastman Inhibitor OABH-EF, which is an oxalylbis(benzylidene-hydrazide), available from Eastman Chemical (Kingsport, Tenn.), and Naugard XL-1, which is a high performance, dual functional phenolic antioxidant/metal deactivator, e.g., 2,2xe2x80x2-oxamido bis-(ethyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, available from Uniroyal (Middlebury, Conn.).
In preparing the formulation of the present invention, the metal deactivator is added to the thermoplastic polyurethane by melting the polyurethane and mixing in the metal deactivator. Once the metal deactivator is added, then the carbonyl iron powder is mixed into the melt. The melt is then allowed to cool to room temperature (about 20 to 25xc2x0 C.).
Although the order of addition is critical (metal deactivator before carbonyl iron), the particular method is not critical. There are many other ways to prepare the formulation of the present invention in addition to the melt process described above. For example, powders of the metal deactivator and polyurethane may be ground together, followed by addition of carbonyl iron powder.
In any event, once the three components are mixed together, the resulting formulation is formed into glue sticks of the requisite dimensions to be used in a glue gun. As is well-known in this art, the glue gun heats the glue stick, thereby melting the formulation, which is then easily applied as necessary. The molten RAM flows onto the site being repaired.
Once applied as a melt to the desired surface, the formulation undergoes rapid hardening in cooling to room temperature. The solidified formulation may be reworked with a soldering iron to smooth the formulation, which, upon re-solidification, may be further smoothed by sanding.
The RAM formulation of the present invention is intended for repairs of small areas, such as seams; repair of large areas is not presently contemplated.